added links in public where an index note item refers to the ref_id of an archival object
added a selenium test
Add public_prefix to the note link generated to a component ref_id
handle cases where archival_objects don't have resources
improved method of extracting resource id
moved the index note reference linking logic from the archival object controller to the notes mixin
Two tweaks that I couldn't resist:

  * Use a 'ref' to store the URI of the record referenced by an index note.  No
    real difference in behaviour, but it would allow people to resolve the
    record if they ever wanted to, and that's how we model inter-record links
    in other places.

  * Tweak the notes mixin to be agnostic about the types it's dealing with.
    Should allow it to work for other tree-based records too.
Actually, make note_index refs untyped
Merge pull request #628 from hudmol/index-note-links

Index note links